Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

262 S. Prachuab.17 MAY 1955EMrliCUALKE WEN9 5" 100" 105" EFigure 6. Historical earthquakes of magnitude 2 7.0 during the period 1900 to 1983.are the most prevalent. It is presumed that there are more earthquakes havingmagnitude 5 4.5, that would be detected only by a larger seismic network.An empirical relationship has been established by Gutenberg and Richter (1954).This relation usually is called the magnitude-frequency relation and is expressed aslog N = a - bM, (1)where N is the number of earthquakes having magnitude M, and a and b areconstants depending on the period of observation and the tectonic region.Using the equation given by Utsu (1967):whereb = 0.4343/ M - M,, (2)is the mean which is c Mi/ c i, and M, is the lowest magnitude.